Add doc to direct developers to enable features for iced::time::every

[fogarecious]

Online document of Iced has the function iced::time::every. However, this function cannot be called when using thread-pool backend.

This PR adds doc to explicitly direct developers to enable appropriate features. This does not change any compiled code.

[fogarecious]

This comment provides a more detailed explanation of the PR.

The online document has a function named iced::time::every. As outlined in the document, there are no constraints on using this function, particularly requiring any specific features. However, when calling the function as shown below:

struct MyApp;

impl iced::Application for MyApp {
    type Executor = iced::executor::Default;
    type Message = iced::time::Instant;
    type Theme = iced::Theme;
    type Flags = ();

    fn new(_flags: Self::Flags) -> (Self, iced::Command<Self::Message>) {
        (Self, iced::Command::none())
    }

    fn title(&self) -> String {
        "".into()
    }

    fn update(&mut self, _message: Self::Message) -> iced::Command<Self::Message> {
        iced::Command::none()
    }

    fn view(&self) -> iced::Element<'_, Self::Message, iced::Renderer<Self::Theme>> {
        "".into()
    }

    fn subscription(&self) -> iced::Subscription<Self::Message> {
        iced::time::every(std::time::Duration::from_secs(1))
    }
}

and including the following dependencies in the Cargo.toml file:

[dependencies]
iced = "0.10.0"

it leads to a compiler error:

error[E0425]: cannot find function `every` in module `iced::time`

To address this issue, the simplest solution is to enable one of the following features: tokio, async-std, or smol.

This PR includes documentation for the function, explicitly instructing developers to enable the mentioned features.

While this may potentially be a cargo doc issue, it proved to be quite confusing for me, requiring a significant amount of time to identify the source of the error. I hope this PR can assist other developers facing the same problem.

(If you believe the problem is not significant, please inform me, and I will close the PR.)

[hecrj]

Thanks! I made some changes to use doc_cfg instead.